1. Field of the Invention
The present invention relates to a process for reducing corrosion and fouling in seawater, cooling waters, process waters and permits the use of carbon steel in seawater utilization; the process is also applicable to well, river, lake, saline, brackish and waste waters. The same process is also useful in reducing SO.sub.x /NO.sub.x /CO.sub.2 -containing flue gas emissions and in reducing disposal problems of some industrial wastes.
2. Description of the State of the Art
Cooling Water, Process Water Applications and Seawater Utilization
Problems arising in seawater utilization in the industry are connected with corrosion, scale, fouling and biofouling. Corrosion is mainly caused by the action of dissolved oxygen, chlorides and sulfates. Scale arises from precipitation of seawater (SW) dissolved salts; to this is to be added fouling from corrosion products deposition . On fouling, plankton also plays a role, in that it deposits and attaches to metal surfaces in the form of "biological gelatine", such gelatine working as a binder of deposited materials. On the deposits, which adheres on metallic surfaces, come and install biofouling colonies, formed by superior marine animals that "live without moving" realizing there their entire living cycle. Among such organisms there are some that contribute to corrosion, specially if the surfaces are iron based. SW corrosion on carbon steel is high (10-50 mpy (1 mpy=0.0254 mm/year), and even 150-200 mpy in still SW), and it is then mandatory to use costly metallurgies (generally the copper and titanium based ones) in the systems that make use of SW.
Nowadays no chemical corrosion inhibitor is utilized to combat SW corrosion on carbon steel: zinc and chromates, the actually most effective water corrosion inhibitors, would be too costly and not as effective as they are in ground waters; moreover, due to the excessive dosages to be employed, they would be in any case unacceptable from an environmental standpoint. The same for phosphorus based chemicals.
For biofouling control, chlorine is of normal use, but a chlorine residue of 0.2 ppm (legal limit e.g. in Italy) is not sufficient to the scope. Sometimes even chlorine residues of 2 ppm are non effective, e.g. in SW cooling towers, against all forms of marine life.
Scale and fouling control are achieved limiting seawater the possibility to concentrate over a certain amount . As a matter of fact SW is normally utilized in once-through systems, in which SW has a huge volume; there are a few systems which recycle SW in cooling towers, mainly to avoid an out of specification discharge temperature (typical concentration numbers for seawater in cooling tower applications are 1.1-1.2). No chemical means are used in once-through SW cooling towers to combat scale and fouling, as this would be too costly and of minor efficiency.
A common feature of all SW cooling systems is that they utilize costly metallurgies (generally the copper based ones, like admiralty, cupro-nickel, aluminum brass) to face the huge corrosion problems that would arise with the more low cost metallurgies (e.g. carbon steel).
The same with SW distillation desalination plants (e.g. MSF). In SW evaporators, due to process high temperatures, beyond the already cited corrosion and CaCO.sub.3 deposition problems, CaSO.sub.4 deposition must be taken into account (which forms a very hard deposit, difficult to remove). This all limits the overall system efficiency. In such desalination plants, pretreatment of SW begins with screening at the intake pit to remove debris. The SW is then acidified to neutralize the carbonates and bicarbonates prior to passing into a packing tower, where the combination of vacuum and stripping steam deaerates and removes the other noncondensable gases such as CO.sub.2. An oxygen scavenger and caustic are added to the water out of the deaerator Caustic is added to adjust pH after acid addition. To avoid acid addition a deposit control agent can be added. SW decarbonation can also be accomplished by aeration (eventually after acidification). Other softening methods include e.g. reverse osmosis, electrodialysis ion exchange, ion selective membrane, d.c. potential application, etc.
In today's state of the art, besides that, not working sea-lines are SW filled to avoid corrosion. Such SW is therefore inhibited with an oxygen scavenger (150 ppm about) and filming biocides (e.g. 200 ppm of cetyltrimethylammonium bromide) this results in high costs (more than 2.1 $/m.sup.3 at today's prices) and environmental problems for treated water disposal. Speaking about static SW, SW firefighting systems suffer huge corrosion problems that may even endanger system security.
There are a number of chemical compositions in the art able to effectively treat waters but, to our knowledge, only few have been claimed to be effective in SW applications (some antiscale in MSF desalination plants), and none are effective in SW utilization in cooling towers.
Waste Waters Utilization
In MgO or Mg production from seawater (SW), SW is first filtered to remove debris and then treated with little amounts of lime to precipitate bicarbonates as CaCO.sub.3, which is separated from SW before reaction. Alternatively, SW is decarbonized by acid reaction (up to pH&lt;4) and then degased to remove formed CO.sub.2. Such treated SW is then reacted with CaO (up to pH&gt;11) to precipitate Mg as Mg(OH).sub.2. In some cases NaOH is also added. The formed slurry is transferred to a decanter, where precipitate separates while exhausted SW overflows and after pH correction is disposed in the sea.
In today's state of the art, SW that has reacted with lime and/or NaOH and from which has been precipitated mostly of present Mg has no further utilization and becomes a waste to dispose of.
In chlorine-caustic soda production by NaCl containing salt solution electrolysis, mercurous waters, after equalization and demercurization, are disposed following environmental regulations. Such waters are constituted by cells wash waters, straining waters from brine and apparatuses, condensed waters coming from cells (which contain chlorine): they all came in contact with Na.sub.2 CO.sub.3 and/or Ca(OH).sub.2 and/or NaOH and contain free chlorine, besides having pH&gt;11 (11.7).
In today's state of the art such waters have no further utilization and become a waste that needs to be disposed.
Other kinds of waste waters find no further application than disposal. Some attempt has been made concerning municipal waste water reuse as cooling tower make-up water but not all of them have been successful, as application problems depend on water characteristics.
Solid Waste Utilization
In industrial processes, sludges produced during operation are today a major environmental problem in that most of them are disposed by discharge in controlled lands (landfilling).
In particular, such disposal is made for sludges from softening plants, chlorine-caustic plants, Mg/MgO plants, drilling operations, etc. Sludges are normally conditioned with organic flocculants, dewatered by centrifugation and/or filtration and landfilled after inertization. Disposal is then a costly operation and, in any case, is believed to be a big operational and environmental problem. Some sludges (e.g. drinking water purification sludges) are utilized in agriculture or in the manufacture of cement. Some processes for recovering chemicals (e.g. lime) from water treatment sludges are reported. Other water and wastewater sludge cakes reuse processes include: freeze-thaw method, sand-drying beds and carbon filtration-adsorption.
Sorbent Materials for Reducing NO.sub.x, SO.sub.x, and CO.sub.2 from a Gaseous Mixture
In the following description what is said for SO.sub.2 can be extended to SO.sub.x.
In today's state of the art, SO.sub.2 emission can be controlled with various desulfurization processes, of which are of industrial interest those using sorbents like lime, limestone, MgO, Mg enhanced lime or limestone, seawater scrubbing. Sorbents can be injected as a slurry in an appropriate scrubber or directly in the boiler as a fine powder. Spray-towers (that are high liquid/gas ratio (L/G) scrubbers) can also be used Typical sorbent ratio in spray-towers are 1.1-1.6 Ca(OH).sub.2 /SO.sub.2 (1.1 if solid recycle is performed). Suitable flue gas desulfurization temperatures ranges from 110 to 130.degree. C.
Desulfurization units can be composed of a prescrubber (which must guarantee a minimum 80% solid particulate abatement) and of a scrubber in which is injected sorbent slurry. In the prescrubber ashes and other pollutants are removed from flue gas, with the scope of ensuring required purity to commercial CaSO.sub.4 and Mg(OH).sub.2, and is accomplished steam flue gas saturation and alogen adsorption.
When possible, SW can be utilized in the prescrubber, but a less pure CaSO.sub.4 is obtained (due to chlorides presence); in such case the produced gypsum can be utilized in a housing, as road ground or in a landfill.
Magnesium enhanced lime profoundly alters SO.sub.2 absorption chemistry. Mg increases slurry adsorption capacity, at least 10-15% more in comparison to that of lime alone. The main advantage is that SO.sub.2 adsorption is governed by the degree of gas/liquid contact in the scrubber, not on solids dissolution, as it does with limestone systems. This enables the process to achieve high removal efficiencies at significantly lower L/G ratios. Also, the chemistry, specifically the higher operating pH, inhibits CaSO.sub.4 formation so little scaling can occur. Mg(OH).sub.2 addition to lime or limestone units reduces oxidation and prevent scaling (allows the desulfurator to operate in undersaturation conditions with regard to CaSO.sub.4 and scale is not formed in the scrubber), and notably increases SO.sub.2 abatement. Due to high sorbent amounts employed, technology costs are high and for the same reason transport problems arise.
In SW scrubbing, for SO.sub.2 adsorption use is made of the natural alkalinity of SW. SW is transported in huge volumes, once-through, in a scrubber where it adsorbs SO.sub.2. In such process a water pH lowering takes place, with consequent SW disposal problems. Adsorption efficiency only depends on SW volume. As a matter of fact, SO.sub.2 reactions in SW are: EQU SO.sub.2 +H.sub.2 O+1/2 O.sub.2 .fwdarw.SO.sub.4 =+2 H.sup.+ EQU HCO.sub.3.sup.- +H.sup.+ .fwdarw.CO.sub.2 +H.sub.2 O EQU SO.sub.2 +H.sub.2 O.fwdarw.H.sup.+ +HSO.sub.3 .fwdarw.2 H.sup.+ +SO.sub.3 =
in which reaction completion is favored by H.sup.+ removal due to SW alkalinity and pH. High pH in fact favors SO.sub.2 adsorption. Referring to SO.sub.2 adsorption efficiency it is then evident that SW natural dissolved salts have a negligible impact--what SO.sub.2 reactions concerns--in comparison to SO.sub.2 water dissolution.
A desulfurization method well known in the art uses a lime or limestone water slurry . This is a widespread technology in the industry.
NO.sub.x removal is achieved, e.g. , by thermal or selective non-catalytic reduction (SNR) processes, which rely on injecting ammonia, urea or other nitrogen-containing compounds into the flue gas in a temperature regime of 870-1300.degree. C. to reduce NO.sub.x to water and nitrogen. Catalysts can enhance NO.sub.x removal.
Description of the Relevant Literature
Few corrosion inhibitors are reported to effectively reduce seawater corrosion, and these relate to metallurgies different from carbon steel. A great number of corrosion inhibitors are reported to be effective on other types of water. EPA 451434 gives a list of commercial corrosion inhibitors and dispersants to be used in waters different from seawater.
FR 2,656,877 describes picolinic and isonicotinic acid inhibitors for Al and Al alloys in contact with Fe and Fe alloys in seawater.
Benzotriazole, tolyltriazole, mercaptobenzothiazole and 2-(5-pentylamino)benzimidazole at concentrations of about 10 ppm effectively inhibit Cu corrosion in areated seawater (Chemical Abstracts 107: 12568 g).
Pol. PL 124,465 reports corrosion prevention on surfaces in circulation cooling systems supplied with softened water; cooling systems using softened water are protected from corrosion by passivation and inhibition with H.sub.3 PO.sub.4 esters, nonorganic/nonoxidizing chemicals, fatty acid surfactant and polyethylene oxide. Suitable mixtures for passivation and inhibition contain mixed esters of H.sub.3 PO.sub.4 with triethanolamine and MeOH 250 and 25, NaNO.sub.2 500 and 50, fatty acid surfactant 250 and 25, and polyethylene oxide 20 and 20 mg/L respectively.
EPA 375,587 reports a method to prevent incrustation of the salts, as well as erosion and formation of microorganisms in an apparatus for distillation of SW. NaCl is added to SW for the purpose.
There are a number of deposit control agents reported to be effective on seawater. Polycarboxyl type antiscalants are reported to be effective in MSF desalination plants. Can. CA 1,158,595 describes an antiscalant for seawater evaporators made by an alkali metal or NH.sub.4.sup.+ salt of a 3-4:1 acrylic acid-Me acrylate copolymer having a molecular weight of 1000-3000.
There are a number of biofouling control agents reported to be effective on seawater. Seawater macrofouling is for example combated with butyltin compounds.
Concerning waste reuse, to our knowledge, few patents are issued in the field of the present invention. CN 1,041,340 describes a method for softening water by lime treatment including reclamation of sludge generated in the process with the steps of: mixing water with lime slurry at controlled concentration to precipitate CaCO.sub.3 and separate CaCO.sub.3 and water by overflowing water from the top of the reactor, filtering softened water and adjusting it to pH 7.5 for use, discharging the precipitated CaCO.sub.3 from the reactor, draining the CaCO.sub.3, decomposing the CaCO.sub.3 in a rotary kiln and cooling the formed quicklime, and preparing the lime slurry from the quicklime for reuse.
It is also reported adipic acid byproducts utilization for enhancement of flue gas desulfurization.
U.S. Pat. No. 4,834,955 reports a composition for inhibiting corrosion and deposits in cooling towers and gypsum scaling in flue gas desulfurization systems which comprises a polyacrylate, polymaleic arhydride, and a phosphonate, and may include tolyltriazole and soluble zinc. The composition is suitable for corrosion and scale control in a combined process in which a portion of the cooling water blowdown is used for preparation of wash water for the mist eliminator in a flue gas desulfurization system. The cooling water contains 0.2-100 ppm active inhibitor composition, and the supplemental wash water contains 0.01-20 ppm active composition. A suitable inhibitor is added at 100 ppm to cooling water.
For SO.sub.x, NO.sub.x, CO.sub.2 emission control, a number of processes have been reported as effective. The more widespread commercial technologies make use of lime and/or limestone and/or Mg compounds for SO.sub.x emission control and ammonia and/or urea for NO.sub.x. Particular types of flue gas desulfurization processes are reported in U.S. Pat. No. 4,708,855, EPA 250,866, EPA 250,878. JP 62,278,119 describes a process in which a CaCl.sub.2 -containing Mg(OH).sub.2 water slurry, which is formed by adding Ca compounds (e.g. CaO, CaO+MgO, Ca(OH).sub.2) to seawater, is treated with MgSO.sub.4 aqueous solution to form CaSO.sub.4 2H.sub.2 O which is separated from the aqueous slurry. The purified Mg(OH).sub.2 water slurry is used for flue gas desulfurization, and the MgSO.sub.4 aqueous solution in this method is the recycled effluent from the desulfurization process. Thus, 18 ton/h seawater was mixed with CaO water slurry at pH 10.4-10.6 in a reactor, then the solid impurities (e.g. sand and CaCO.sub.3) were separated by sedimentation. The remaining water slurry was overflowed and mixed with coagulant to obtain a Mg(OH).sub.2 water slurry, which was then treated with 3% MgSO.sub.4 aqueous solution to form a precipitate of CaSO.sub.4 2H.sub.2 O. The purified Mg(OH).sub.2 water slurry was contacted with flue gas for desulfurization and the resulting MgSO.sub.4 aqueous solution was recycled for purification of the Mg(OH).sub.2 water slurry Ger. Offen. 2,801,279 describes a process in which scrubbing solutions containing sulfates and hydroxides of Na, K or NH.sub.4.sup.+ are regenerated with lime. In JP 03 52,623 SO.sub.x is removed from boiler flue gases by wet scrubbing with lime slurry in an absorption tower comprising means for pumping seawater and lime into a mixing tank to obtain an aqueous slurry. To prevent corrosion, the concentration of COD, BOD and dissolved O.sub.2 in the feed SW are preferably controlled at .ltoreq.1, .ltoreq.1, and .ltoreq.4 ppm respectively. and the process piping materials are preferably made of SUS-316 L steel.
About modification of "classic" sorbents, one is to note WO 92/1509, in which smectite clay suspension in water containing Na.sub.2 CO.sub.3 are reacted with stoichiometric amounts of soluble alkaline earth metal to form an alkaline earth metal carbonate, which after drying is suitable for SO.sub.x removal.
Concerning NO.sub.x removal, NH.sub.3 addition is a common technique. U.S. Pat. Nos. 4,029,752, 4,288,420, 4,400,363, 4,272,497, 4,051,225, 3,900,554, 4,325,713, 4,321,241, 4,853,193 are exemplary patents disclosing ammonia utilization.
Italian patent application No. ME92A000002 to MEG S.n.c. filed 30.03.92 discloses a process for decreasing metals corrosion and fouling in systems utilizing seawater, well, lake, river, waste waters and mixtures thereof.
Italian patent application No. ME92A000006 to MEG S.n.c. filed 27.05.92 discloses a process for removing SO.sub.x, NO.sub.x, CO.sub.2 from a gaseous mixture and sorbents for the scope.
Italian patent application No. ME92A000007 to MEG S.n.c. filed 18.06.92 discloses a process for utilizing waste waters from industrial processes.